The shoulder complex is composed of multiple joints and therefore has many degrees of freedom, and these joints exhibit coordinated motion during upper limb movement (Ludewig, et al., 2009). Specifically, the scapula exhibits upward rotation, external rotation and posterior tilt, and the humerus is externally rotated during upper limb elevation (McClure, et al., 2001). This motion is primarily the motion measured during the scapular plane elevation, but the scapular kinematics vary slightly with the elevation plane (Ludewig, et al., 2009). For example, in sagittal plane elevation, initially the scapula is internally rotated and then externally rotated toward the end range of the elevation, whereas in frontal plane elevation, the scapular is externally rotated from the initial phase of the elevation. Thus, the scapula maintains conformity with the glenohumeral joint by changing its kinematics depending on the elevation plane (van der Helm, 1994, Voight and Thomson, 2000).

Complex scapular motion is controlled by the periscapular muscles (Kibler and McMullen, 2003). The upper and lower trapezius and serratus anterior are particularly important, and their coordinated activity forms a force couple to the scapula (Camargo and Neumann, 2019, Neumann and Camargo, 2019). It has been reported that these muscle activities differ among elevation planes (Alenabi et al., 2016, Reed et al., 2016). However, no studies exist that have quantified the coordinated activity that is important for making force couples and investigated it in combination with scapular motion. Furthermore, in addition to the muscles listed above, there are many other muscles in the shoulder joint, and there are countless combinations of muscle activities to generate specific scapular motion in each elevation plane (Bernstein problem) (Bernshteĭn, 1967). It is unclear how this problem is overcome and coordinated motion of the scapula is achieved.

To properly control this redundant musculoskeletal system, the central nervous system (CNS) is thought to simplify a large number of muscles into a small number of modular structures (synergy hypothesis) (Bernshteĭn, 1967, Safavynia et al., 2011). Muscle synergy have been quantified through the use of detention reduction techniques (d'Avella, et al., 2006), showing that even complex movements of the upper limb can be explained by a small number of fundamental muscle synergy combinations (Saito et al., 2022, Umehara et al., 2021). These findings suggest that the CNS achieve each movement task by flexibly recruiting a small number of muscle synergies with appropriate activation levels.

The purpose of this study was to determine the muscle activities and intermuscular coordination that contribute to changes in shoulder kinematics by the elevation plane. We hypothesized that changes in shoulder kinematics associated with different elevation planes could be produced by modifying the activity levels of a small number of muscle synergies.